1. Field of the Invention
This invention relates to computer circuitry and, more particularly, to methods and apparatus for generating a cursor or other overlay which is clearly distinguishable from the background picture being displayed on a computer output display.
2. History of the Prior Art
In order to facilitate the use of computers, the use of bitmapped graphics displays and pointing devices has become prevalent. A pointing device, such as a mouse, provides input signals to control precisely the position of a cursor on the output display to enable some action to take place at the position. With a simple black and white display, the position of the cursor may be easily discerned by the operator if a line of pixels surrounding the cursor is of opposite color value to the cursor. Thus, if the background is black and the cursor black, a thin white line surrounding the cursor allows the cursor to be easily discerned. If the cursor is black and the background is white, the thin white line of pixels merges with the background and disappears, but the cursor remains easily discernible.
A more difficult problem arises where the thin line may not be placed around the cursor for any of various reasons such as the shape or type of the particular cursor. In such a situation, in order to discern the overlay, it is typical to render the overlay in a color which contrasts with the background pixels.
With a black and white display it is typical to obtain a color value for the cursor which contrasts with the background by simply inverting the signal defining the background color at each particular pixel position; for example, if a zero binary value indicates white and a one indicates black, inverting either value produces the opposite value. However, if the output display is not a simple black and white display but a gray scale display, providing a discernible cursor (or other overlay) is more difficult. The shades of gray of pixels (often referred to herein as color values) produced on a gray scale monitor are typically indicated by a code of eight bits (one byte). An inversion of these values does not necessarily produce a shade of gray which is discernible from the background. For example, if the typical digital code indicating a middle shade of gray (i.e., 0111 1111) is inverted, a shade (i.e., 1000 0000) immediately adjacent that initial shade is produced, a shade barely discernible from the initial shade. A similar problem exists in attempting to produce discernible overlays for color displays where eight bits of binary information are used to define each of the red, green and blue components producing the color of the pixel.
Another problem encountered in attempting to discern a cursor from the background relates to the ability to describe accurately the shape of the cursor where its size is quite small with respect to the number of pixels used. The small size of the cursor makes its display much more subject to the jagged edges typical of diagonal lines on an output display of low resolution. To date no arrangement for solving this problem has been found for display overlays.